Appliances, such as dishwashers, washing machines, clothes dryers, and the like are typically driven by electric machines. A motor drive provides power from a source, such as a household power outlet, to the electric machine. The household power outlet typically supplies A/C power at a line voltage (such as 115V) and a line frequency (such as 60 Hz).
Line voltage transients, or surges, can occur due to lightning strikes and other sources. Voltage surges may reach up to 6000V. Residential electrical appliances are designed to withstand these power surges. Some motor drives incorporate surge protection circuits that limit damage due to power surges. One surge protection circuit includes a line to neutral metal oxide varistor (MOV) and a neutral to ground MOV in the motor drive circuitry. The MOV's clamp the surge voltages.
Appliances typically undergo insulation testing, which requires 1200V to 1800V to be applied to the electric machine through the motor drive. This high voltage causes conduction of traditional MOV-type surge protectors that are incorporated in the motor drive which prevents satisfactory testing. As a result, a jumper circuit is used during insulation testing to disconnect the surge protection circuit. The requirement of connecting and disconnecting the jumper circuit adds additional cost and time to the manufacturing process.
Another surge protection circuit employs spark gaps in the circuit board of the motor drive. The breakdown voltage of spark gaps, however, is adversely impacted by dirt and humidity variations. Spark gaps are further subject to carbon accumulation and metal displacement from electrodes into the spark gap area, which limits their useful life.